The present invention relates to chemical processes involving the vaporisation of a liquid phase organic compound which is to undergo chemical reaction in the course of the process. The invention has particular application to organic compounds which can be vaporised satisfactorily at atmospheric pressure but become prone to degradation when vaporisation is effected at elevated pressure, with consequent elevation of the boiling point (or bubble point) of the compound.
For instance, when trichloroethylene is vaporised at elevated pressures such that the boiling point thereof is in excess of 150xc2x0 C., degradation products are formed which cause fouling of heat exchange surfaces thereby impairing performance (vaporisation efficiency). Also, such degradation of trichloroethylene leads to the formation of undesirable chemical species which are vaporised and pass into the process utilising the vaporised trichloroethylene.
Trichloroethylene is used in the production of compounds such as 1,1,1,2-tetrafluorethane by reaction thereof with a stoichiometric excess of HF in the presence of a fluorination catalyst. Where this process is carried out at elevated pressure (which is advantageous since the plant for carrying out the process can be made compact), problems arise in achieving satisfactory vaporisation of the trichloroethylene.
According to a first aspect of the invention there is provided a process for carrying out a chemical reaction involving a first organic compound which has been vaporised from the liquid phase under conditions which render the liquid phase form of the pure compound prone to degrade. characterised in that vaporisation of said first compound from the liquid phase is effected using a second compound in such a way as to suppress degradation of said first compound.
The chemical reaction may be one involving both compounds as reactants as in, for example, the fluorination of trichloroethylene using hydrogen fluoride. However, we do not exclude the possibility of the chemical reaction being one which does not involve the second compound as a reactant (primary or otherwise), e.g. the chemical reaction may be one involving the isomerisation of a vaporised chlorocarbon in the presence of an isomerisation catalyst, e.g. the isomerisation of HCFC 123a to produce HCFC 123.
The invention in all aspects thereof defined herein has particular application to the vaporisation of halocarbons (which term is used herein to include halogenated compounds which incorporate hydrogen) and more specifically chlorocarbons such as chloroethenes, e.g. trichloroethylene and perchloroethylene, and chloroethanes such as 1,1,1,2-tetrachloroethane and 1,1,2,2-tetrachloroethane.
The term xe2x80x9cdegradation productxe2x80x9d as used herein is to be construed as including within its ambit, derivatives of said organic compound which are not desired in the context of the particular chemical reaction. For instance, in the case of 1,1,2,2-tetrachloroethane, the chemical reaction may be one in which it is to be converted to 1,1,2-trichloro-2-fluoroethane but may be prone to convert to trichloroethylene if subjected to high temperature whilst in the liquid phase (e.g. as a result of coming into contact with a hot metal surface) and, in this instance, for the purposes of the present invention trichloroethylene may be regarded as a degradation product of 1,1,2,2-tetrachloroethane.
When used in suppressing degradation of said first compound, the second compound may be in the liquid or gas phase. In the former case, the degradation suppression may be effected by co-vaporising the first compound in admixture with the second compound.
According to a second aspect of the present invention there is provided a process for carrying out a chemical reaction at elevated pressure, comprising vaporising under elevated pressure conditions a liquid phase organic compound which forms one of the reactants and is prone to degrade when vaporised under such conditions, and reacting the vaporised compound with at least one other reactant in the gaseous phase under conditions which permit the reaction to proceed, characterised by modifying the boiling characteristics of the organic compound such that vaporisation can be effected at reduced temperature thereby preventing or substantially reducing the production of degradation products.
Preferably the boiling characteristics of the organic compound are modified by co-vaporising the compound in admixture with another compound which may form one of the reactants. The compound which is added to said organic compound is preferably one which provides a mixture capable of forming a minimum boiling azeotrope. In general, the second compound will have a boiling point lower than that of the first compound.
According to another aspect of the present invention there is provided a process for effecting fluorination of a chlorocarbon compound at elevated pressure, comprising vaporising the compound under elevated pressure conditions at which the boiling point of the compound per se is such that vaporisation of the compound per se is accompanied by degradation of the compound if vaporised at that boiling point, and reacting the vaporised compound with a fluorinating agent in the gaseous phase under conditions with permit the reaction to proceed, characterised by the selection of hydrogen fluoride as the fluorinating agent and effecting vaporisation of the chlorocarbon by co-vaporising a mixture of the chlorocarbon compound with hydrogen fluoride whereby the boiling temperature of the chlorocarbon compound/HF mixture is such that vaporisation thereof is accompanied by substantially less degradation of the chlorocarbon compound.
Preferably the chlorocarbon compound is a chloroethene such as trichloroethylene or perchloroethylene, or a chloroethane such as 1,1,1,2-tetrachloroethane or 1,1,2,2-tetrachloroethane.
In yet another aspect of the invention there is provided a process for carrying out a chemical reaction involving a halocarbon in its vapour phase, in which prior to the reaction the halocarbon is vaporised under elevated pressure conditions at which the boiling point of the compound per se is such that vaporisation of the compound per se is accompanied by degradation of the compound if vaporised at that boiling point characterised in that the halocarbon is co-vaporised in admixture with hydrogen fluoride whereby the boiling temperature of the halocarbon compound/HF mixture is such that vaporisation is accompanied by substantially less degradation of the halocarbon compound.
Typically the reduction in degradation is at least 50% by weight, i.e. the amount of degradation products produced is at least 50% by weight less than would occur if the pure halocarbon is vaporised at its boiling point under said elevated pressure conditions.
In one embodiment of the chemical reaction may comprise isomerisation of the halocarbon compound, e.g. the isomerisation of 1,2-dichloro-1,2,2-trifluoroethane (HCFC 123a) in its vapour phase to the more stable 1,1-dichloro-2,2,2-trifluoroethane (HCFC 123).
In another embodiment of the invention the chemical reaction may comprise the fluorination of a chlorocarbon such as a chloroethane or a chloroethene in which case vaporisation of the chlorocarbon by co-vaporisation with HF and the HF may also constitute the fluorinating agent. For example, the chloroethene may comprise trichloroethylene and the reaction is one involving fluorination of trichloroethylene. We have found that, whilst vaporisation of trichloroethylene at elevated pressure would normally result in the production of degradation products, co-vaporisation of the chloroethylene in admixture with HF has a marked affect on the boiling point to the extent that even small additions of HF allow efficient vaporisation of the chloroethylene under elevated pressure conditions without any significant degradation of the chloroethylene.
Where the chloroethene comprises trichloroethylene, the elevated pressure at which fluorination is carried out is typically in excess of that at which the boiling point of trichloroethylene is 150xc2x0 C., more preferably 180xc2x0 C. Usually the trichloroethylene will be vaporised at a temperature somewhat below that of the process stream into which it is introduced with the advantage that the trichloroethylene so introduced has a cooling affect on the process stream.
Certain aspects of the invention as defined above are primarily concerned with effecting vaporisation of a liquid phase organic compound so that the resulting vapour can then be fed as such to a process utilising the vapourised form of the compound. The following aspects of the invention are concerned with processes in which the organic compound is fed in its liquid phase form to the process in which it is to be utilised, in circumstances where the compound is to be vaporised by contact with a hot gaseous stream in the proximity of hot surfaces such that, if liquid phase droplets of the compound are allowed to reside in contact with such surfaces, the compound is prone to degrade.
In particular, the following aspects of the invention concern a chemical process involving contact between a hot gaseous stream and an organic compound which, when in the liquid phase, is prone to degrade if allowed to reside in contact with a hot surface, said method comprising injecting said organic compound in its liquid phase into a conduit through which the hot gaseous stream flows in such a way that the liquid atomises and vaporising the liquid droplets so formed by contact with the hot gaseous stream.
The gaseous stream and/or a gaseous compound present in the gaseous stream can be utilised to prevent the degradation of organic compound as a result of contact of said organic compound in its liquid phase with surfaces heated by said gaseous stream.
Such utilisation of the gaseous stream and/or said gaseous compound may take various forms.
In accordance with one feature of the invention the organic compound is injected as a liquid phase stream together with a second compound as a gas phase stream into a conduit through which the hot gaseous stream flows so that the first organic compound is atomised at least in part by interaction between the injected streams, the liquid droplets so formed being vaporised by contact with the hot gaseous stream.
Thus, the atomisation of the liquid phase organic compound may be effected at least in part by the interaction between the injected streams. The interaction between the hot gaseous stream and the injected liquid phase stream may also serve to effect atomisation.
This feature finds application for instance in circumstances in which the throughput of the hot gaseous stream may vary for example between a full load condition and a turn-down condition where the throughput is substantially reduced, e.g. 75% or less, and often 50% or less, than the throughput under full load. In such a case, the throughput in turn-down conditions may be insufficient to secure efficient atomisation of the injected liquid phase stream by the hot gaseous stream. The interaction between the injected streams may therefore be used to compensate for loss of efficiency in atomisation in turn-down conditions.
The second compound, injected as a gas phase stream, is preferably one of the components forming the process gas stream. For example, in fluorination processes, the injected gas phase stream may comprise hydrogen fluoride.
In another feature of the invention, the hot gaseous stream and/or gaseous phase compound may be controlled in such a way as to either substantially eliminate contact between heated surfaces and the droplets or at least reduce the residence time of contact between said heated surfaces and the droplets to such an extent that vaporisation of the liquid occurs before any significant degradation can develop.
This may be implemented for instance by injecting said organic compound as a liquid phase stream into a conduit through which the hot gaseous stream flows, the first organic compound being atomised at least in part by interaction with the hot gaseous stream and the droplets so formed being vaporised by contact with the hot gaseous stream; and injecting a second compound as a gas phase stream into the conduit downstream of the location of injection of the first compound to compensate for the pressure drop in the wake of the injected stream of said first compound and thereby prevent deposition of said liquid droplets on the conduit wall immediately downstream of the location of injection of the first compound.
In a further development the reduction in residence time may be implemented by controlling the gaseous stream and/or said gaseous compound so as to xe2x80x9csweepxe2x80x9d the heated surfaces and thereby clear any droplets settling thereon and resuspend them in the gaseous stream.
Thus in accordance with this development of the invention there is provided a chemical process involving contact between a hot gaseous stream and an organic compound which, when in the liquid phase, is prone to degrade if allowed to reside in contact with a hot surface at or above a predetermined temperature, said process comprising:
atomising the organic compound by injection thereof into a conduit through which the hot gaseous stream flows, the conduit wall or walls being heated at least in part by the hot gaseous stream to at least said predetermined temperature;
vaporising the liquid droplets so formed by contact with the hot gaseous stream as the liquid droplets are transported by the hot gaseous stream downstream of the location of injection; and
introducing a gas phase component into the conduit at a location downstream of the location of injection of the organic compound so as to to form a boundary layer at the conduit wall or walls for reducing or preventing deposition of the liquid droplets on the conduit wall or walls.
The gaseous component used to develop the boundary layer may be constituted by part of the hot gaseous stream diverted from the main flow and re-introduced into the conduit at said downstream location. Alternatively or additionally the gaseous component may comprise an auxiliary gas which itself may participate in a subsequent chemical reaction with the organic compound; for instance where the chemical reaction is one involving fluorination of a halocarbon, the auxiliary gas may comprise HF.
In accordance with another aspect of the invention there is provided a liquid injection device for the introduction of liquid phase organic compounds into a process gas stream, comprising:
a conduit section for coupling into, or forming part of, a conduit for the process gas stream;
a Venturi arrangement housed within the conduit section and including a throat section and a diffuser section downstream of the throat section;
means for partitioning the process gas flow into a first stream which passes through the Venturi arrangement and a second stream which by-passes at least part of the Venturi arrangement;
a plurality of nozzles for injecting the liquid phase compound into the Venturi arrangement at, or in close proximity to, the threat section whereby the liquid is atomised to produce droplets which are vaporised by the process gas stream; and
means for introducing the second stream into the Venturi arrangement at a location downstream of the nozzles in such a way as to create a moving layer of gas over the surfaces of the Venturi arrangement whereby unvaporised liquid droplets migrating towards such surfaces are prevented from residing in contact therewith.
Other aspects and features of the invention will become apparent from the following description of specific embodiments of the invention and also the appended claims.